Optical communication systems are capable of transmitting optical communication signals thousands of kilometers without electrical terminations or regeneration. For this reason, optical monitoring points are available at each network element, usually at its input and output, in order to perform measurements like optical signal to noise ratio, OSNR, power, frequencies for maintenance, automatic performance reporting, etc., as required by ITU-T Recommendation G.697. It is possible to exploit optical monitoring points to access the optical signal of interest and mess it up with broadband or frequency selective optical noise. Monitoring points comprise an optical splitter that splits off a certain portion of the optical signal, typically in the range of 1% to 10%, to form an optical monitoring signal, which are typically then delivered to a monitoring port of the network element, which can be connected to an optical spectrum analyser to perform the measurements indicated above, or to an internal photodiode for power measurement.
Optical communication signals transported by the optical communication system can be accessed through a monitoring point of the first kind and this poses a security problem because the communication signal is susceptible to an optical jamming attack. Optical jamming is a known technique to disrupt an optical communication signal in which broadband or narrowband optical signals, such as noise or tones, are inserted into the optical communication system to intentionally deteriorate optical communication signals carrying traffic. This can be done by using optical monitoring ports as inputs for the attacking optical signal. Forms of optical jamming include causing intra-channel cross-talk, which uses an attacking optical signal at the same frequency as the optical signal to be disrupted, and inter-channel cross-talk, which uses an attacking signal at a different frequency, and causing gain competition, which uses a higher-powered attacking signal to deplete the optical power assigned to weaker optical communication signals on different wavelengths.
The problem of making optical monitoring apparatus secure against attacks by optical jamming has attracted some interest in the technical literature due to the continuous trend to extend the application of optical technologies towards the end user. The proposed solutions assume that an optical jamming attack will take place in some way and propose ways to detect that it is happening or to mitigate it. For example, a wavelength allocation scheme to mitigate the effect of an inter-channel optical jamming attack by limiting its maximal propagation in an optical domain is reported by N. Skorin-Kapov et al, “Limiting the propagation of intra-channel crosstalk attacks in optical networks through wavelength assignment”, Optical Fiber Communication Conference 2009, JWA65.